This invention relates to the production of synthetic polymeric material and, more particularly, to an apparatus useful in producing such material.
Condensation polymers such as the polyamides were initially produced by batch processes in autoclaves, but continuous polymerization processes have been developed and are generally preferred for present day commercial operation. A continuous polymerization process is described by Taylor in U.S. Pat. No. 2,361,717. Taylor discloses continuously passing an aqueous solution of a diamine-dicarboxylic acid salt at super atmospheric pressure and at amide-forming temperatures continuously through a long tubular reactor divided into separate sections which are maintained at different conditions of pressure and temperature. The initial temperature-pressure conditions are such that the formation of steam is prevented and the rate of travel of the solution is such that a major portion of the salt is converted to polyamide. The reaction mass is then further subjected to amide-forming temperatures at pressures permitting the formation of steam, to remove water from the reaction composition as steam until the composition consists essentially of polyamide.
As a further simplification of Taylor's process the prior art discloses designs in which concentrated nylon salt solution is pumped under pressure directly into a stepped diameter flash tube in which water is flashed off as the nylon is polymerized. The deficiency of these designs is that in addition to steam flashing off in the flash tube a large amount of diamine is also vaporized since the salt solution is exposed to high temperatures and energetic boiling before the diamine has reacted. Since there is no subsequent step for reabsorbing the diamine, a large diamine loss occurs when the steam plus vaporized diamine are subsequently separated from the polymer. Of course, a major concern in the continuous polymerization of nylon is to avoid diamine loss, or, at least, to control it to a constant quantity, regardless of through put. This is essential not only because the diamine is an expensive intermediate but also because it is essential to maintain constant amine end content of the polymer in order to maintain constant molecular weight and acid dyeability.
It is, therefore, an object of the present invention to provide an improved process for continuous preparation of nylon whereby steam is disengaged from polyamide forming salt solution i.e., the reaction mass while the latter continues to polymerize in a stepped diameter reactor tube. The invention provides for recombining unreacted diamine, which has been vaporized with the steam in the reactor tube, with the reaction mass.